There's really nothing to it. Either you just register the .php3 file extention to use the php4 engine, or you rename all the files from *.php3 to *.php.
There are some, very few, changes in the syntax. You can check out www.zend.com to see what kind of code that need to be changed. In most cases no changes whatsoever are needed. Simply recompile PHP and enjoy the new and faster engine.

It is possible to use both php3 and php4. This is described in the installation documentation for php4.